I'm not sure about switching providers domestically, but I have been told by Cingular's technical support that the phones are not sim-locked for the purposes of using foreign sims during international travel.

The SIM card tells the network who you are; in essence it is your id to the network. Put your sim card in another phone and you are charged for calls made on that phone. It is easily accessible in the back of the treo.

In addition, it stores phone numbers (and other stuff we don't seem to be able to get). I programmed my wife's sim card by putting it in my phone, copying phone number to it using the sim book program on the treo, then put it back in her motorola phone, which was way easier than entering the numbers. (The automatic phone number entry on VS's website is apparently vaporware.)

A major advantage of the sim card approach (as opposed to the nonremovable equivalent on CDMA phones, that require electronics expertise or a service visit to change) is that one can get a disposible sim card for europe that is essentially a prepaid calling card, and your phone works.